1. Field of the Invention
The present invention particularly relates to a WDM (Wavelength Division Multiplexing) optical transmitter using a Fabry-Perot laser.
2. Description of the Related Art
Typically, in the realm of WDM light source a number of such sources have been developed including DFB (Distributed FeedBack) laser array, MFL (Multi-Frequency Laser), spectrum-sliced light source, wavelength-locked Fabry-Perot laser with incoherent light, and R-SOA (Reflective Semiconductor Optical Amplifier), etc. The spectrum-sliced light source increasingly evolving performs spectrum-slicing on a broadband optical signal using a WGR (Waveguide Grating Router), and thus provides users with a large number of WDM channels. Therefore, there is no need for the spectrum-sliced light source to use neither a light source of a specific lasing wavelength nor a wavelength stabilizer, etc. Recently, new LEDs (Light Emitting Diode) have proposed: SLD (Super Luminescent Diode), FB (Fabry-Perot) laser, fiber amplifier light source, and ultra-short pulse laser, etc. as a spectrum-sliced light source.
The wavelength-locked Fabry-Perot laser with incoherent light performs spectrum-slicing on a broadband optical signal created from an incoherent light source such as an LED and an optical fiber amplifier light source, using an optical filter or a WGR (Waveguide Grating Router); the spectrum-sliced broadband optical signal is injected into a Fabry-Perot layer having no isolator, which adapts the resultant wavelength-locked signal to a signal transmission mode. If a spectrum-sliced signal of more than a predetermined output level is applied to the Fabry-Perot laser, the Fabry-Perot laser generates only a wavelength equal to that of the received spectrum-sliced signal. The reflective semiconductor optical amplifier (R-SOA) applies incoherent light to the SOA (Semiconductor Optical Amplifier), and adapts the amplified optical signal to a signal transmission mode.
The manufacturing process of the DFP laser array and the MFL is complicated, and elements that select a correct wavelength and stabilize the wavelength so as to implement a WDM scheme are costly. Although an LED and an ultra LED proposed as a spectrum-sliced light source have good optical bandwidth and are low-priced elements, they have low modulation bandwidth and low output level. Therefore, they are appropriate as a light source for use with an upstream signal having a modulation rate lower than that of a downstream signal. Although the Fabry-Perot laser is a low-priced high output element, it has a narrow bandwidth, such that it is impossible to provide a large number of wavelength division channels. In case of modulating and transmitting a spectrum-sliced signal at a high speed, the Fabry-Perot may have performance degradation due to mode partition noise. Although the ultra-short pulse light source has a wide spectrum bandwidth as a light source and has coherent characteristics, it not only has low stability of an oscillated spectrum but also a narrow pulse width of several picoseconds (ps), such that it its implementation is difficult. In lieu of such light sources, a spectrum-sliced fiber amplifier light source has been recently proposed. It provides a user with a large number of wavelength-division high output channels by performing spectrum-slicing on the ASE light created from the optical fiber amplifier. However, this spectrum-sliced light source must use a high-priced external modulator such as a LiNb03 to allow individual channels to transmit different data.
The wavelength-locked Fabry-Perot laser with incoherent light directly modulates a Fabry-Perot laser according to a data signal, resulting in increased economical efficiency of data transmission. However, in order to allow the Fabry-Perot laser to output a wavelength-locked signal suitable for high-speed long distance transmission, a high-output incoherent light signal having a wide bandwidth must be applied to the Fabry-Perot laser device. Further, absent temperature control a slight change in external temperature results in the Fabry-Perot laser changing mode. Therefore, the Fabry-Perot laser escapes from a wavelength equal to that of a received spectrum-sliced signal, such that it cannot be adapted as a WDM light source using a wavelength-locked Fabry-Perot laser. An external temperature controller (i.e., a TEC controller) is indispensable to adapt such a wavelength-locked Fabry-Perot laser as a WDM light source, and there is a need for an additional wavelength stabilization circuit to stabilize the light source and its wavelength, resulting in a high cost assessed to a subscriber. As a result, a WDM passive optical network (PON) is not commercially viable due to the high financial costs assessed to the subscriber. To solve this problem, a wavelength-locked Fabry-Perot laser transmission scheme having no temperature control process must be developed. The reflective semiconductor optical amplifier's (R-SOA) manufacturing process is complex and costly, and has limitations in modulation rate, resulting in difficulty in its commercial use.
In conclusion, the aforementioned conventional wavelength-locked Fabry-Perot laser with incoherent light has a variety of advantages, but it is ineffective under conditions of temperature variation, resulting in difficulty in its commercial use.